The present invention is directed towards a method of producing alumina trihydrate crystals from an alumina trihydrate recovery process stream.
Production by the Bayer process involves the digestion of bauxite at high temperatures and pressures in caustic soda liquor, producing a saturated sodium aluminate solution (pregnant liquor) containing an insoluble ferruginous residue called “red mud”. In the Sinter process, bauxite is combined with lime and heated to about 1200° C. prior to leaching with caustic soda liquor to generate a sodium aluminate liquor containing insoluble “sinter mud”. Mud slurries generated in the above processes are treated with flocculants to flocculate and separate the muds from the pregnant liquor by gravity settling in thickener vessels (settlers). After settling, the clarified liquor (overflow) is removed from the top of the settler. At this point, the Sinter process often requires another step wherein a desilication additive such as lime is added to the overflow liquor to remove soluble silica from the liquor. This slurry is treated with flocculants and fed to a desilication settler to remove insoluble desilication products. The liquor is then further purified in a filtration process in order to remove suspended fine solids and other impurities.
The purified pregnant liquor—an example of an alumina trihydrate recovery process stream—is then cooled and seeded with fine alumina trihydrate crystals or neutralized with CO2 gas in a precipitation process to produce alumina trihydrate as gibbsite crystals, followed by calcination to produce the final alumina product. In the Bayer process, precipitation of alumina trihydrate from supersaturated caustic aluminate solutions is the rate limiting step, taking up over half of the residence time in an alumina refinery. Precipitation does not take place under ideal conditions because the digestion of bauxite ore in refinery “spent” liquor results in a solution supersaturated in alumina, and which also contains significant amounts of organic and inorganic impurities. Precipitation is accelerated by the use of seed alumina trihydrate crystals.
Bayer process operators optimize precipitation to maximize yield while still obtaining high quality product having a target crystal size distribution. It is desirable to produce relatively large crystals as this facilitates subsequent processing steps. A large percentage of fine crystals (i.e., below 45 micrometers) are undesirable. However the presence of some fine crystals may be desirable for seeding purposes. The yield and properties of the alumina trihydrate crystals can be significantly affected by the process conditions used, such as temperature, residence time, and the nature of the seed crystal used, and these conditions can vary from plant to plant.
A crystal growth modifier (CGM) can be added to the alumina trihydrate recovery process stream to impose a deliberate modification of the alumina trihydrate crystals. A modification generally used is a reduction in the proportion of fines, and therefore, an increase in the average alumina trihydrate particle size. Crystal growth modifiers can be used to control particle size and strength. Not only must product quality crystals (≧45 micrometers) be produced, but sufficient seed crystals (<45 micrometers) are also needed to promote precipitation. Crystal growth modifiers can also enhance agglomeration by combining and cementing smaller particles. Crystal growth modifiers can also suppress or control primary nucleation (generation of new particles) and secondary nucleation (generation of new particles on surfaces of existing particles). A crystal growth modifier can modify the crystal particle size distribution, allowing the user to use a lower fill temperature and higher seed charge. Crystal growth modifiers can also be used to affect the morphology of oxalate crystals that often co-precipitate in the alumina trihydrate precipitation circuit.
Extensive efforts have been invested into finding effective crystal growth modifiers and methods of their use in optimizing crystal particle size. Many crystal growth modifiers (e.g., C18-fatty acids) require the addition of an oil or secondary surfactant to aid in dispersion of the CGM into pregnant liquor. Added oil or surfactant increases the impurity load in the liquor, negatively impacting precipitation yield, and may cause discoloration of the alumina trihydrate, which is highly undesirable.
Because of the organic content of Bayer liquor (predominantly humic substances), it has a natural tendency to foam. Foaming of the liquor is aggravated by the mixing steps in the Bayer process. Foaming is especially a problem after clarification (separation of the red mud) and during precipitation. The amount of pregnant liquor cannot be maximized in vessels partly filled with foam, and therefore maximum product throughput cannot be obtained. Foam also poses a safety hazard in that overflow can expose workers to high levels of caustic, which can cause severe chemical burns. Since foam is an insulator, reduction in foam can improve heat transfer efficiency. Reduction of foam can reduce scaling in precipitators and improve operation of alumina trihydrate classification systems due to reduced alumina trihydrate retention in foam.
In view of these factors, a way to economically reduce the generation of fine particles in the precipitation of alumina trihydrate is desirable. In particular, the method should provide a decrease in percentage of crystals having a volume average diameter of less than about 45 micrometers. The crystal growth modifier employed should be effective at low doses (i.e., less than about 100 milligrams per liter of pregnant liquor), and should be substantially free of ancillary oils or surfactants, thereby minimizing contamination and discoloration of the alumina trihydrate crystals. Moreover, foam generation in the method should also be minimized.